68 Wisconsin Academy of Sciences, Arts, and Letters. 



son and that such an enzyme is not present or is inactive in the 

 latter part of the growing period as indicated by the fact that 

 hemicellulose is deposited in both the wood and bark at that 

 time. Sanio 137 found that in Pinus silvestris lignification did 

 not occur in spring wood until after the deposition of the secon- 

 dary thickening had been completed, that it began at the angles 

 of the cells and then involved the radial walls and later the tan- 

 gential walls. In the summer wood, however, the primary walls 

 were found to have lignified before the deposition of the secon- 

 dary thickening began, and it occurred in cells whicii were only 

 a few removed from the cambium. The final composition of 

 the cell walls of spring and summer wood seem also to differ, for 

 according to Wieler, 138 the walls of spring wood contain a lower 

 percentage of cellulose than those of summer wood. 



If the deposition and lignification of cellulose are in any way 

 dependent upon enzymotic action, there must be at least two 

 enzymes concerned because the two processes appear to be inde- 

 pendent of each other as indicated by Sanio 's observations. It 

 is evident that either of the processes- would necessarily impede 

 or check further enlargement of cells differentiating from the 

 cambium. It, therefore, appears permissible to assume that the 

 enzymes involved in the solution of hemicellulose and the tardi- 

 ness of the lignification process in spring are important factors 

 in permitting the development of larger wood cells in spring 

 than those produced in summer, when the cellulose dissolving 

 enzymes are inactive and lignification occurs so quickly after a 

 cell is formed that in some cases it takes place even before sec- 

 ondary thickening has begun. The experiments by Jost and by 

 Lutz also give support to the idea that radial growth is largely 

 controlled by enzymotic activities which are somehow dependent 

 upon the process of terminal elongation. Perhaps the enzymes 

 concerned are liberated or activated in enlarging and bursting 

 buds in different parts of trees and are carried downward in the 

 metabolized food, or possibly enzymes produced in the enlarging 

 buds simply initiate certain activities which are transmitted 

 without the further aid of the enzymes as was assumed by 



"' Sanio, K. Anatomie der gemeinen Kiefer (Plnus silvestris L, ) 

 Jahrb. Wiss. Bot. 9: 66-68. 1873. 

 "• 1. c. 



